Correlation of Cooperatively Localized Rearrangement on the “Fluidized Domain”of Polymers to Their Nonexponentially Viscoelastic Behavior and Lifetime at Double Aging Processes II: Estimation of Long-term Mechanical Behavior and Lifetime of Polymeric Mate

以四种不同类型高聚物(半晶聚丙烯、聚甲基烯酸甲酯、交联环氧树酯和炭黑填充丁苯橡胶)的短时标应力松弛和蠕变测试实验数据作为实例,按照两套应力松弛模量和蠕变柔量普适对比态方程,从理论组合主直线上直线外推了四种高聚物的长时标力学行为和寿命,并将其外推预测值同理论计算值和实测值进行了对比. 结果证实四种高聚物的外推预测值均同理论计算值和实测值很好地符合. 又讨论了它们的尺寸稳定性、负荷承载性,失效寿命和失效形变量等对老化时间的依赖性. 最终证实该从理论组合主直线上来直线外推长时标力学行为和寿命的方法是有效可行的.     

Correlation of Cooperatively Localized Rearrangement on the “Fluidized Domain” of Polymers to Their Nonexponentially Viscoelastic Behaviors at Double Aging Processes (I): A Set of Reduced Universal Equations on the Stress Relaxation Modulus and Creep Comp

基于α-和β-微区、微晶、交联和缠结链组网络结构和非线性粘弹性复相涨落模型,提出了双重(物理和力学)老化下不同尺寸和末端向量高分子链组的流动化微区内关联性局部化重排动力学新理论,它能把初级α-微区的降解作用和次级β-微区的再生效应对剪切速率、尺寸大小和温度依赖性有机结合起来．然后用统计力学和动力学结合法计算了三种不同类型高聚物在等温等压双重老化过程中多元链组的总构象改变自由能,证实了它起源于多元链组的尺寸大小、形状和体积的变化．推导出了它们单向拉伸下的本构方程、应力松弛模量和蠕变柔量表征式以及它们相对应的两组对比态普适方程．结果表明该两组对比态下应力松弛模量和蠕变柔量普适方程均具有同K-W-W伸展指数函数完全一致的形式．最终又建立了该两组对比态普适方程中三个分子参数同老化条件、温度和高聚物起始结构之间的相关性．     

金属材料失效分析的新方法

提出了一种预测金属材料失效的新方法,该失效准则考虑了应力三轴度和Lode角参数的影响。将金属材料分为ēfef和ēf≤ef两类,其中ēf和ef分别定义为应力三轴度η=1/3、Lode角参数ξ=1(轴对称应力状态)和应力三轴度η=1/3、Lode角参数ξ=0(平面应变状态)时的真实应变。另外只需要两个常见的实验(光滑圆杆拉伸试验和纯剪试验)数据就可以确定失效准则的参数值。将该新失效准则与文献中报道的诸多材料在不同加载条件下的实验数据进行对比,结果吻合较好。     

材料参数拟合方法对弹靶侵彻仿真的影响

弹靶侵彻仿真中材料参数对计算结果有着至关重要的影响。为寻求一套适用于弹靶侵彻仿真计算的材料参数拟合方法,借助前期开展的靶板材料动态力学性能试验、靶板材料断裂试验,通过不同拟合方法依次得到不同的JC本构模型及失效模型参数,依据试验建立有限元计算模型,将数值计算结果与试验结果进行对比。结果表明:(1)对于同一材料的力学性能试验,采用不同的拟合方法可得到不同的JC本构、JC失效参数,二者会对弹靶仿真结果造成一定影响;(2)在不考虑温度软化项的前提下,采用高应变率作为参考应变率进行拟合能更加准确地表征材料在高应变率下的应力-应变关系,更加适用于弹靶侵彻强瞬态、高应变率作用过程仿真;(3)对于同一JC本构模型,采用平均应力三轴度拟合的JC失效模型较采用初始应力三轴度拟合的JC失效模型所得战斗部剩余速度计算结果偏小,仅采用拉伸试件结果拟合的JC失效模型较采用扭转、拉伸试件结果拟合的JC失效模型所得战斗部剩余速度计算结果偏小。     

激光辐照下预加载CFRC层合板断裂行为实验研究

针对激光与机械载荷联合作用下碳纤维/环氧树脂增强复合材料（CFRC）层合板失效时间的预测需求,实验研究了不同激光功率密度（70～210 W/cm2）、不同预应力水平（拉伸强度的50%和70%）、不同光斑尺寸（拉伸试件宽度的70%和100%）下2 mm厚层合板的失效机理,获取了不同影响因素对断裂时间的影响规律。结果表明:预加载层合板失效机制为迎光面环氧树脂基底材料热解、纤维氧化断裂,背光面剩余结构偏脆性断裂;在预应力一定条件下,试件断裂时间与辐照激光功率密度成指数规律;预应力水平对断裂时间影响显著。     

含有缺陷3C-SiC陶瓷拉伸性能的分子动力学模拟

SiC纤维增韧SiC基复合材料(SiC_f/SiC)由于其优越的性能而成为新一代核能系统重要候选材料之一.材料中的缺陷会使材料的力学性能发生变化,本文运用分子动力学程序LAMMPS模拟计算了分别含有空位、微空洞和反位替代三种缺陷的3C-SiC结构体系沿[100]方向的拉伸变形过程,原子间相互作用采用Tersoff多体势描述.通过模拟得到不同缺陷体系的应力—应变曲线和拉伸过程中体系能量,通过分析应力-应变曲线,得到了不同缺陷体系的杨氏模量、断裂应变、拉伸强度随缺陷"浓度"的变化关系,最后分析了3C-SiC拉伸断裂机理.研究结果表明,空位和微空洞对杨氏模量、拉升强度的影响类似,都是随着缺陷"浓度"的增加而减小,反位替代缺陷使体系的杨氏模量随缺陷"浓度"的增加而增大.     

环氧树脂玻璃钢的动静态拉伸力学特性

为系统地研究环氧树脂玻璃钢在静、动态拉伸载荷作用下的力学性能，采用材料测试系统和分离式霍普金森拉杆对材料进行拉伸试验，获得0.001～0.1 s^-1及1 128～1 840 s^-1应变率下的应力-应变曲线和相应的力学参数。结果表明，动态加载下环氧树脂玻璃钢的应变率增强效应较为明显。为此，引入动态增强因子描述环氧树脂玻璃钢在高应变率下力学性能的增强。采用扫描电镜对损伤断面进行观测，发现动态加载下纤维束平整断裂，而非静态加载下纤维拔出失效。相较于静态加载，动态拉伸载荷作用下玻璃钢的基体-纤维界面断裂韧度更高。基于环氧树脂玻璃钢在动态拉伸下的力学响应，引入宏观损伤累积量，建立一种考虑损伤的非线性拉伸本构模型。拟合结果表明，该模型整体上可以反映环氧树脂玻璃钢在动态拉伸载荷作用下的力学响应。     

基于微孔洞长大惯性机制的动态拉伸断裂模型构建

 采用圆柱体胞模型分析方法,对球形微孔洞在不同加载应变率条件下的动力学响应行为进行了有限元分析,计算结果表明：在微孔洞稳定增长阶段,惯性对微孔洞的快速增长起着关键性作用,其它因素的影响基本可以忽略,微孔洞半径增长率与平均应力的平方根成正比。提出了一个微孔洞增长惯性机制的损伤度演化方程,结合逾渗软化函数描述微孔洞聚集行为,从而构建了一个新的动态拉伸断裂模型,并通过自定义材料模型子程序,把断裂模型嵌入LS-DYNA程序中,对无氧铜平板撞击层裂实验进行了数值模拟研究,计算结果与实验结果的比较令人满意,初步检验了新模型的实用性。     

含有断裂缺陷的复合材料壳体的力学行为

利用湿法缠绕制备了玻璃纤维增强环氧树脂基复合材料壳体,通过拉伸、双悬臂梁和三点端部开口弯曲试验对该复合材料层合板进行基本力学性能评估,得到强度和刚度参数并用于有限元模拟中。同时,在Abaqus中建立了含有不同深度断裂缺陷复合材料壳体的三维渐进损伤有限元模型,预测内压作用下壳体的力学响应。试验和模拟结果表明:复合材料主向拉伸强度为(222.7±18)MPa,弹性模量为39.39 GPa,Ⅰ型和Ⅱ型断裂韧性层间强度分别为(4.67±0.24)和(4.98±0.26)kJ/m~2。随着内压增大,Mises应力也不断增大;当断裂缺陷在最深层(接近内压的第一层,深度为18 mm)时,Mises应力最大;当内压为0.3 MPa时,Mises应力高达28.8 MPa,且周向应变小于纵向应变。     

纳构件加工后的力学特性

"采用分子动力学模拟的方法,运用镶嵌原子模型,研究了经过刻划后的单晶铜纳构件在不同拉伸速度条件下的力学行为. 通过原子位图、缺陷原子透视图、径向分布函数及应力-应变关系研究加工后纳构件在拉伸负载作用下的变化特性,并与理想单晶铜纳构件进行对比分析．模拟结果表明,加工后的纳构件的屈服强度较理想纳构件的屈服强度有明显下降,屈服强度随着刻槽深度的增加而下降,而且屈服强度对刻槽方向和拉伸速度敏感;纳结构在拉伸负载作用下,其应力应变关系出现了双峰形式,即工作硬化现象,二次屈服后表现为Z字形逐波下降形式．刻划深度、刻划     

Shifting selectivity of collinear volume holographic storage

A paraxial equation for calculating pixel-based 2-D shifting selectivity of collinear holographic storage system is proposed. The equation shows that the broader reference pattern makes high shifting selectivity. In addition, both amplitude and phase modulation make no obvious change in shifting selectivity so shifting selectivity is not affected through advanced design of reference pattern. Besides, a simplified formula based on a point signal is proposed, which shows a clear physical insight that the shifting selectivity is the square of Fourier transform of the intensity distribution of reference pattern and does not depend on thickness of the disc when a point signal is applied.     

Alloying Effect on the Creep Mechanism and Creep Resistance of Polycrystals in the Concepts of Physical Mesomechanics

The effect of different slightly soluble alloy additions on the creep and individual creep-component behavior of lead at T = 0.5 T _cr under controlled conditions (high-purity polycrystals, the same grain size) is investigated on the basis of the concepts of physical mesomechanics. The additions used are shown to reduce the creep rate under these conditions. The effect of the slightly soluble alloy additions to the polycrystal on the steady-state creep rate is produced through grain-boundary sliding and localized-deformation banding near grain boundaries.     

Broadening and shifting of the 321-, 325- and 380-GHz lines of water vapor by pressure of atmospheric gases

Three spectral lines of the main water molecule isotope in the ground vibrational state located near 321, 325 and 380 GHz were studied at low pressures and room temperature using spectrometer with radio-acoustic detection of absorption. Self-, N₂- and O₂-pressure broadening and shifting parameters of these lines have been precisely measured. A number of parameters, in particular pressure shifts, were obtained for the first time. Complementary study of the 325-GHz line by resonator spectrometer at atmospheric pressure validated the low pressure experiment data and allowed measurement of the 325-GHz line intensity. Obtained results are discussed in comparison with previous experimental and theoretical data.     

Theories of Creep in Ceramics

Mathematical models that have been proposed for creep in ceramics are described. Emphasis is on models involving grain boundary motion (sliding or flow). In Lifshitz models the crystalline grains elongate with strain; the elongation results from diffusion, slip, or solution and precipitation. In Rachinger models the grains do not elongate during creep. The sliding strain can be accommodated by viscous flow of a glassy phase at the grain boundaries, or if there is no boundary glass by diffusion or slip in superplastic models. Sliding of a glass-free boundary can result in cavitation, cracking, or formation of boundary dislocations or triple point folds.

Most models of ceramic creep at high temperatures predict a steady state (stage II) creep rate that depends on the applied stress, grain size, and temperature. A general equation for the creep rate as a function of these factors, as well as the elastic modulus and a diffusion coefficient, is used to compare models. The models give different exponents for the functional dependence of creep rate on grain size and strain and different temperature dependencies. These differences are compared in tables, and the main mechanistic features of the models are described in the text.

The purpose of this review is to describe creep models rather than to compare them with experimental results or to select the most applicable models. There are few critical experimental tests that allow selection of the most accurate models; such experiments are suggested as the next step in choosing between the models for specific experimental results.     

Creep of beryllium. I

Creep characteristics of beryllium have been determined in the temperature range 600–800°C and the stress range 0.25–5 kgf/mm². The rate of the process is controlled by the Herring —Nabarro mechanism in the range of stresses less than 1 kgf/mm². The creep activation energy (39±1 kcal/mole) hence agrees with the energy of self-diffusion. The creep rate for stresses greater than 1 kgf/mm² is determined by the simultaneous progress of dislocation creep and slip, where the slip contribution grows with the increase in stress. An approximate picture of the deformation mechanisms of creep is constructed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 68–73, March, 1976.     

Creep in oriented thermoplastics

Sheets of oriented low-density polyethylene possessing transverse symmetry were prepared by simple tensile drawing at room temperature. The degree of anisotropy was varied by varying the draw ratio. Classic elasticity theory shows that five constants are necessary to characterize the deformation behavior of an elastic material possessing such symmetry. The time-dependent equivalents of these constants have been determined from the simultaneous measurement of longitudinal and lateral strain during tensile creep of specimens cut at 0, 45, and 90 deg to the draw direction. Special creep apparatus, able to handle small, flexible specimens, was developed for this study. The shear compliance obtained from the tensile creep studies was in good agreement with the value obtained from torsional creep measurements on a 0-deg specimen, for the entire range of draw ratios. The contraction measurements have been used to provide additional evidence for deformation mechanisms suggested by the longitudinal strain measurements and, when combined with tensile measurements, have allowed the volume changes occurring during tensile creep to be calculated.     

Creep of multidirectionally fibre-reinforced composites

A model for the creep of metal matrix composites multidirectionally reinforced by short fibres is proposed. The reinforcement is described by the effective stiffness tensor of a multidirectional arrangement of continuous fibres and the internal damage of the composite during creep due to fibre fragmentation is introduced by assigning a heuristic nonlinear stress–strain relationship to the fibres. Based on the model, the load partitioning between matrix and fibres is computed. The macroscopic creep behaviour is simulated for composites exhibiting different fibre orientation distributions and different heuristic nonlinear stress–strain functions. The computational results rationalize the creep behaviour of multidirectional fibre-reinforced composites. For a two-dimensional random orientation distribution, a good qualitative match between simulation and experimental results is obtained for compressive loading and for in-plane tensile loading. For loading normal to the reinforcement plane, the model overestimates the creep resistance. In this case, the formation and growth of cavities seems to govern the creep deformation of the composite.     

Introduction in the band theory of molten salts

The electrochemical properties of molten salts are considered in the frame of band theory for ideal crystalline dielectrics. It turned out that metal impurities at low concentration in them generate the local electron levels in the wide band gap (>4 eV) of molten salt divided by 50 meV and more. They are occupied by electrons when Fermi level is approached to them from below. Then, the minor metal impurities fall out the molten salt when the electron occupation of their energy levels in the band gap achieves their solubility (<0.1 ppm) in the molten salt. At the same time, the given RedOx-potential of molten salt can be maintained without chemical additives but forced shifting of Fermi level by the electrochemical cell with one electrode strongly polarized and the other in equilibrium with this molten salt.